1. Field of the Invention
The present invention relates to an electrical contact including an internally oxidized silver-oxide material which has high electrical conductivity and excellent electrical contact characteristics over a long period of time in the form of a compact element, that is, one which exhibits high welding resistance and high wear resistance and is suitable for an electromagnetic relay which is made smaller in size.
This application claims priority from Japanese Patent Application No. 2003-289820 filed on Aug. 8, 2003, and Japanese Patent Applications Nos. 2003-401296, 2003-401297, 2003-401298 and 2003-401299 filed on Dec. 1, 2003, the contents of which are incorporated herein by reference.
2. Background Art
Various electromagnetic relays are used as functional components of automobiles, office equipments, etc.
The electromagnetic relay 100 is constituted, for example, from an electromagnet 101 including an iron core 111 and a coil 112, an armature lever 102 having a substantially L-shaped section, a movable contact spring 141 and a stationary contact spring 142 that are provided above the armature lever 102, and electrical contacts 151 and 152 fixed at one end each of the movable contact spring 141 and the stationary contact spring 142 opposing each other, as shown in schematic longitudinal sectional views of FIG. 6A and FIG. 6B.
At least a part of the electromagnet 101 is covered by a yoke 103, with an insulator 106 provided on the top surface of the yoke 103. Other ends of the movable contact spring 141 and of the stationary contact spring 142 are secured on the insulator 106. A return spring 143 is provided above the stationary contact spring 142, while one end of the return spring 143 is secured on the insulator 106. A contact drive card 107 is provided in contact with the movable contact spring 141 between the armature lever 102 and the return spring 143.
When an electric current flows in the coil 112 of the electromagnet 101, one end 102a of the armature lever 102 is attracted by the iron core 111 as shown in FIG. 6B. Thus, the armature lever 102 swings around an armature hinge 102c, so that the other end 102b of the armature lever 102 causes one end 141a of the movable contact spring 141 to move upward via the contact drive card 107. Consequently, the electrical contact 151 fixed at the distal end of the movable contact spring 141 and the electrical contact 152 fixed at the distal end of the stationary contact spring 142 make contact with each other so that current flows therethrough, resulting in the active state of the relay.
When the flow of the current in the coil 112 of the electromagnet 101 is stopped, the electrical contacts 151 and 152 separate from each other so that the relay rests in the inactive state shown in FIG. 6A.
In the case of the electromagnetic relay 100 having the structure described above, it is used under the conditions of 14 VDC for the power voltage and rated current of 20 to 30 A, if it is used in an automobile. In this case, the electrical contact usually has a rivet-shape measuring 3 to 5 mm in diameter.
Recently, automobiles and office equipment have been rapidly acquiring versatile functions and high performance, while growing smaller in size and lighter in weight. Accordingly, the electromagnetic relays that are functional components of automobile, office equipment, etc., are also becoming smaller in size. Thus, the electrical contacts used in the electromagnetic relay have been becoming smaller in size, and are required to have a head diameter in a range from 1.5 to 2.5 mm in the case of a rivet-shaped one.
Even when made smaller in size, the electromagnetic relay must operate under the same conditions as those of the conventional ones, that is, under conditions of 14 VDC for the power voltage and rated current of 20 to 30 A in the case of automotive application. Thus, the current density flowing in the electrical contact per unit area becomes much higher as the contact is made smaller.
Various materials have been proposed and commercialized for the electrical contacts used in the electromagnetic relay having the structure described above. Among these, internally oxidized silver-oxide material that has a metallographic structure such that ultra-fine grains of Sn-based oxides and ultra-fine grains of In-based oxides are precipitated in an Ag matrix (to be described later) is attracting much attention.
There is the internally oxidized silver-oxide material which is made by subjecting an Ag alloy having a composition consisting essentially of, by weight (percentages are by weight), 4.5 to 10% Sn, 0.1 to 5% In, and 0.01 to 5% Bi, with the balance being Ag and unavoidable impurities, to an internal oxidation treatment under the conditions of maintaining at a temperature ranging from 650 to 750° C. in an oxidizing atmosphere for 15 to 30 hours, as disclosed in U.S. Pat. No. 4,680,162.
There is also the internally oxidized silver-oxide material which is made by subjecting an Ag alloy having a composition consisting essentially of, by weight (percentages are by weight), 5 to 10% Sn, 1 to 6% In, and 0.01 to 0.5% Ni, with the balance being Ag and unavoidable impurities, to an internal oxidation treatment under the conditions of maintaining at a temperature ranging from 650 to 750° C. in an oxidizing atmosphere for 15 to 30 hours, as disclosed in Japanese Patent Application, Second Publication No. S55-4825.
There is also the internally oxidized silver-oxide material which is made by subjecting an Ag alloy having a composition consisting essentially of, by weight (percentages are by weight), 3 to 12% Sn, 2 to 15% In, and 0.1 to 8% Cu, with the balance being Ag and unavoidable impurities, to an internal oxidation treatment under the conditions of maintaining at a temperature ranging from 650 to 750° C. in an oxidizing atmosphere for 15 to 30 hours, as disclosed in Japanese Patent Application, First Publication No. S51-55989.
There is also the internally oxidized silver-oxide material which is made by subjecting an Ag alloy having a composition consisting essentially of, by weight % (percentages are by weight), 4 to 11% Sn, 1 to 5% In, and 0.05 to 4% Te, and, if necessary, 0.03 to 0.5% Ni, with the balance being Ag and unavoidable impurities, to an internal oxidation treatment under the conditions of maintaining at a temperature ranging from 650 to 750° C. in an oxidizing atmosphere for 15 to 30 hours, as disclosed in Japanese Patent Application, First Publication No. H04-314837.
The electrical contact including the internally oxidized silver-oxide material described above for the use in electromagnetic relay, however, has relatively low electrical conductivity. Thus, when the electrical contact is including the internally oxidized silver-oxide material in a small size, greater heat generation occurs between the contacts, and which leads to softening of the contacts. As a result, the contacts have significantly deteriorated welding resistance and wear resistance, eventually reaching the end of their service life in a relatively short period of time.